A technique for separating liquid/liquid mixtures and liquid/solid mixtures includes filtration (Perry Robert H. Perry's Chemical Engineers' Handbook, 6th ed., pp. 19.65-19.89, 1984) (this hand book is referred to as the “Perry document” hereinafter). In the filtration, a mixture to be separated (liquid-solid mixture or liquid-liquid mixture) is supplied to a filter layer made of, for example, a porous material (such as diatomaceous earth) or a fibrous material, and a liquid is passed through the filter layer due to differential pressure (such as centrifugal force) while solids are captured in the filter layer. Alternatively, a low-viscosity liquid is passed through the filter layer, while a high-viscosity liquid and/or solids are captured in the filter layer and separated. With respect to the low-viscosity liquid and the high-viscosity liquid, a liquid and a liquid immiscible therewith (hereinafter referred to as an “immiscible liquid”) (a plurality of immiscible liquids may be included) in a liquid-liquid mixture are regarded as the low-viscosity liquid and the high-viscosity liquid, respectively, by a relative comparison of viscosities.
1. Filtration and Recovery of Captured Material
In the filtration, it is a difficult problem to improve recovery and a recovery rate (=recovery amount/capture amount) of substances captured in the filter layer. The filtration also includes separation by a membrane (such as ultra filtration membrane). Methods for improving the recovery rate of captured substances include a method using an organic solvent (such as hexane) (Perry, pp. 15 1-15 20). However, this method may cause deterioration of the captured substances. In addition, the solvent is generally recovered by an evaporation method, thereby increasing the cost of the method (a large quantity of heat energy is required for evaporating the solvent).
2. Meltable Filter Medium
It is considered that filter crystals described below can be used as a filter medium for capturing viscous liquid and/or solids. The filter crystals are the followings:
1) Fine crystals (which may be grains), needle/rod-like crystals, dendrite-like crystals, or flake/plate-like crystals produced in a liquid; crystals produced by removing or scraping crystals formed by contact between a cooled solid such as metal and a liquid; or crystals by grinding (such as impact grinding using rotation centrifugal force) or crystals by crushing after their formation.
2) Crystals produced by mixing liquid in gas by low-temperature evaporation, heating evaporation, spraying, or liquid dropping, forming crystals by contact between the resultant mixture and a cooled solid (such as metal) or a solid (such as plastic), and removing or scraping the crystals from the cooled solid or the solid.
3) A crystal group including the crystals described in 1) and/or 2).
The filter crystals are meltable single crystals and/or polycrystals. Possible substances for the filter crystals include materials such as ice. The above-mentioned 1), 2) and 3) are described in the following a) and b).
a) Formation of Filter Crystals in Liquid
It is known from documents below that filter crystals can be formed in liquid by rapid crystallization of single-components liquid or rapid crystallization of multi-component liquid.
With respect to formation of ice crystals from an aqueous solution, Thijssen, H. A. C., A. Spicer ed., Applied Science Pub. LTD., London UK, p. 117-121, 1974 (hereinafter referred to as the “Thijssen document”) discloses the followings:
1. By unidirectional freezing, the ice grows in the form of needles or bars with an irregular cross section perpendicular to the cooled surface.
2. In a liquid, amounts of fine crystals increases (the occurrence rate of crystal nuclei (fine ice) increases) with increasing cooling rates or increasing solute concentrations.
3. Ice crystals increase in size over time.
With respect to freezing of water, Yoshinori F. and Etsuro Y., JASMA Vol. 21, 217-223 2004 discloses that an the amount of dendrite crystals increases with increasing cooling rate.
In addition, PETER V. HOBBS, CLARENDON PRESS OXFORD, pp. 580-581 1974 discloses that dendrite crystals are easily formed from an aqueous solution as compared with pure water.
Examples of materials other than water that can be used for the filter crystals of the present invention include materials such as clathrate hydrates (U.S. Pat. No. 6,237,346 B1).
U.S. Pat. No. 3,845,230 and U.S. Pat. No. 3,320,153 disclose techniques that form a ice-crystal layer from a mixture composed of a mixture to be separated and ice crystals. The U.S. Pat. No. 3,845,230 discloses a centrifugal method to form an ice-crystal layer using a rotating basket. Also it is described that spherical ice crystals are formed by extending a residence time during slow freezing and that fine ice crystals are produced by shortening the residence time during rapid freezing. The U.S. Pat. No. 3,320,153 relates to a technique for separating an oil and a mixture of ice crystals and solidified wax.
b) Formation of Filter Crystals in Gas
A type of formation of filter crystals in a gas includes natural snow.
Filter crystals can be artificially formed in gas as follows: A liquid is mixed in a gas by low-temperature evaporation, heating evaporation, spraying, or liquid dropping, and the resultant mixture is contacted with crystal-forming materials or cooled-crystal-forming materials to produce solidified materials (artificial snow) of the liquid.
3. Filtration
Documents explaining a filter layer and a fluid mixture to be passed through the filter layer according to the present invention are described below. According to these documents, the following matters are found. 1) A filtration is considered as a method capable of separating a liquid-liquid (a high-viscosity liquid and a low-viscosity liquid) mixture by capturing the high-viscosity liquid by a crystal-filter layer and passing the low-viscosity liquid through the crystal-filter layer. 2) A filtration has the function to coalesce immiscible droplets and/or fine solids.
3.1 Characteristics of Filtration
a) Based on research of freeze concentration (separation between ice crystals and concentrate), the Thijssen document describes on pp. 130-132 the following. In a method for separating between ice crystals and a liquid (press, centrifugal filtration, and washing), a permeation rate of the liquid (filtrate amount per unit area and time) is inversely proportional to the viscosity of the liquid and the filter layer thickness (the distance for the liquid to pass) and is proportional to the square of the mean crystal diameter. The Thijssen document also describes that in centrifugal filtration of ice crystals and a liquid, the amount of a liquid remaining in the filter layer is proportional to the viscosity of the liquid, and that the amount of remaining liquid decreases with increasing centrifugal effect (G).
b) In regard to centrifugal filtration, Masao et al., AlChEJ, Vol. 33, pp. 109-120 1987 and Perry document pp. 19.96-19.103 describe the following: The permeation flow rate decreases as the viscosity of a liquid increases and the filter layer thickness (the distance for the liquid to pass) increases. In addition, the permeation flow rate increases with increasing centrifugal effect (G) and with increasing rotation time.
3.2 Coalescing Function of Filtration
In Spielman, L. A. and Goren, S. L., Ind. Eng. Chem., Vol. 62, No. 10, p. 10-24 (1970), U.S. Pat. No. 4,335,001, and S. D. Rege, H. S. Fogler, AlChE Vol. 34, pp. 1761-1772 1988, it is described that the filtration has the function to coalesce fine solids and/or immiscible droplets (such as emulsion) in a mixture to be separated.
In the document of Spielman, L. A. and Goren, it is also described that the larger solids and/or droplets (the larger aggregates), the more easily the solids and/or droplets are captured in a filter layer.
In the present invention, considering the coalescing function (facilitating subsequent separation) of the filter layer, the coalescing function may be used as pre-treatment for separation of the mixture to be separated regardless of the presence of capture in the filter layer.
The document of Spielman and Goren further describes that a difference in permeability occurs between a high-viscosity liquid and a low-viscosity liquid when a mixture of these liquids is passed through the filter layer.